In general, polymer forming technologies are used to manufacture various plastic products by melting polymer grains, i.e. pellets, with electric heat and mechanical friction and then extruding the molten polymer pellets into a mold or dies having a desired shape. Since material costs account for a very high percentage of total costs, foaming technologies have been developed in order to reduce material cost and decrease the weight of a polymer product.
Foaming technologies form a lot of minute bubbles in a polymer product, in which a foaming agent is chemically or physically mixed with a polymer material and heat or pressure is applied to the mixture from outside such that foaming materials can be evaporated from the mixture, thereby forming bubbles inside the product. When the bubbles are generated, most portions of the product are occupied by the bubbles, thereby enabling significant reduction in production costs, decrease in weight of the product, and thermal-insulation performance due to the bubbles.
Typically, a physical foaming process is performed by injecting foaming gas into a molten polymer resin within a cylinder or barrel of an injection or extrusion machine.
The foaming gas is injected into the barrel through a gas injection gate. In this case, if an internal pressure of the barrel filled with the polymer resin is higher than injection pressure of the foaming gas, the polymer resin flows back to the gas injection gate and clogs the gas injection gate.
In this case, the process is delayed until repair of the gas injection gate is finished, and there can be loss of components in the apparatus.
Korean Patent Publication No. 10-2003-0050833 (published on Jun. 25, 2003) discloses a check valve that prevents gas from flowing back to a gas supplier.